Radio frequency amplifier



July 1 1941. J. EVANS RADIO FREQUENCY AMPLIFIER] Filed March 22, 1938 4 Air/vi Patented July 1, 1941 UNITED STATES AT E oFFicE RADEO FREQUENCY AMPLHFIER ware Application March 22, 1938, Serial No. 197,366

7 Ci ims. (Cl. 179171) My invention relates to radio frequency amplifiers, and more particularly to a device for neutralizing a triode radio frequency amplifier which operates satisfactorily over very wide bands of modulation frequencies such as are encountered in television transmitters and the like.

It is well known that precautions must be taken to prevent oscillation in a tuned radio frequency amplifier. Since it is customary to have both the plate and the grid circuits tuned to the same frequency, the capacitance between the grid and plate of the amplifier tube may cause the circuit to oscillate without external excitation. Unless such an amplifier is completely shielded, coupling may also exist between the inductances in the grid and plate circuits.

A number of the well known neutralization methods are satisfactory for frequencies of the order of those utilized in radio broadcasting, and even for moderately high frequencies, but such methods are not satisfactory at frequencies of the order of 50,000,000 cycles per second, and upward, such as are used in television transmission, where modulating frequencies of the order of 2,500,000 cycles per second are used.

I have shown one method of satisfactorily neutralizing a high frequency push-pull amplifier in U. S. Patent 2,091,258, issued August 31, 1937. This invention, like others, is not entirely satisfactory when wide band modulation is used, because the neutralization is effective only at frequencies adjacent to the carrier frequency.

In order to prevent oscillation and degenerative feedback by the process of neutralization, a voltage must be introduced into the grid circuit which is equal in amplitude but opposite in phase to the voltage which is fed back to the grid by the grid-plate capacitance, or by the mutual coupling between the plate and grid tank circuits. At high frequencies ordinary methods of neutralization fail because the leads connecting the neutralizing capacitors cannot be regarded as purely resistive, but act as a complex impedance network. Consequently, the voltage introduced into the grid of the amplifier by the neutralizing device may not be in phase opposition at all, or perhaps only partially so. In addition, the value and phase of the feedback voltage obtained in this manner changes with frequency. This gives rise to another difficulty, for when wide band modulation is desired, as in television transmission, the degree of neutralization changes over the range covered by the carrier and side bands. The result of this is two-fold. In the first place, varying degrees of neutralization cause undesirable attenuation or amplification of particular side band frequencies, and in the second place, oscillation may take place at certain side band frequencies.

The solution of these difficulties will be found in a device which impresses on the amplifier grid a voltage equal in amplitude to the undesired feedback voltage, but opposite in phase for all frequencies covered by the carrier and side bands under the existing band of modulating frequencies. It is, therefore, an object of my invention to provide a method of neutralization which will deliver a neutralizing voltage to the amplifier grid which is, for all frequencies, of proper amplitude and phase. Specifically, this is accomplished by utilizing the phase shift of voltage which takes place from the grid to the plate of a vacuum tube to supply the neutralizing voltage required.

The invention itself will best be understood from the following description when read in connection with the accompanying drawing, and its scope is indicated by the appended claims.

Referring to the drawing, I have shown a preferred embodiment of my invention comprising a push-pull radio frequency amplifier neutralized in the manner of my invention. It is to be understood, however, that my invention is not limited to the embodiment particularly described, but may, for example, be applied to an amplifier comprising but a single triode.

The modulated radio frequency voltage to be amplified is coupled to the amplifier by a coupling coil ll from a source which is not shown. This coil is coupled to an inductor l9 which may consist of a loop, or of several turns of conductor at the mid section of a resonant line 25. This resonant line is preferably a quarter wave length long. It terminates at the grids 1 and I3 of two push-pull triode amplifiers I and 3, respectively. Any conventional method may be used to tune the line to the carrier frequency. For example, the length of the line or the value of inductor I9 may be varied. The electrical center of this circuit is grounded by a grid resistor 2| connected from the mid point of the inductor I9 to ground.

The two cathodes 9 and I 5 of the amplifier tubes are grounded. These cathodes may be of the unipotential, indirectly heated type, or directly heated by a battery or the like, in the customary manner. Anodes 5 and H of the amplifier tubes are connected to the ends of a second resonant line 29 which has an inductor 33 at its mid point which is coupled to an antenna 39-by means of a coupling coil 35. The second resonant line 29 is tuned in the same manner as the resonant line 25 in the grid circuit. The inductor 33 may be a single turn in the mid section of the resonant line 29, or it may consist of several turns, depending on the frequency. Anode potential is supplied to the two anodes and II by a battery 3'1, or the like, connected between ground and the electrical center of inductor. 33.

This completes the connections of a conventional push-pull amplifier. Neutralization of each tube is accomplished, according to my invention, by connecting a neutralizing tube in inverse parallel with each amplifier tube in the following manner, but, to simplify the description, the neutralization of only one tube will be described, as that of the other is identical.

The anode 5 of tube I is connected through a capacitor 59 to the grid 61 of a neutralizing triode 63. Grid bias is obtained from any suitable source, which is not shown, through a radio frequency choke H. The anode B5 of the neutralizing triode is, connected through a coupling capacitor 6! to the grid 1 of the amplifier tube I. Anode potential for the neutralizing tube is obtained from a suitable source, which is not shown, through a second radio frequency choke '73. The cathode 6 of the neutralizing tube is grounded. A similar neutralizing tube 45 is connected in an identical manner to the amplifier tube 3.

The theory of operation of my invention may be explained in detail as follows: It is known that the plate load of a vacuum tube has an effect on its input impedance. The reaction upon the grid of the voltage produced in the plate circuit may cause the input of an amplifier to present, in effect, a negative or a positive resistance to the circuit across which it is connected. When the reactance of the plate circuit is inductive, an effective negative resistance is presented by the grid, and when the reactance of the plate circuit is capacitive, the effective resistance presented by the grid is positive. The negative resistance condition is equivalent to regeneration, and, if

large enough, will cause self-oscillation. The

positive resistance condition increases the effective resistance of the grid circuit and thus causes loss of amplification, or degeneration.

It is also known that, at a frequency just below resonance, the reactance of a shunt tuned circuit is highly inductive, and that it is highly capacitive at frequencies just above resonance. Consequently, in a radio frequency amplifier having a resonant plate circuit tuned to the carrier frequency, the load impedance will be capacitive for one side band and inductive for the other, and the actual phase angle of the side band voltages will depend upon their frequency.

To be completely effective, neutralization must, therefore, compensate for changes in the eifective grid resistance, positive and negative, over the entire range of frequencies covered by the modulation. The device which provides the neutralizing voltage must not itself discriminate against certain frequencies. A vacuum tube is such a device and, when connected in the manner shown, will introduce into the grid circuit of the amplifier tube voltages 180 out of phase with the undesired voltages which are fed back to the grid by the grid-plate capacitance.

I prefer to use neutralizing tubes of the same general character as the amplifier tubes so that their internal electrode construction is similar. Because each neutralizing tube is connected only to one amplifier tube, that is, not cross-connected between two tubes, lead lengths may be reduced to a minimum. In the usual cross-connected neutralization method, the leads are long enough to have an appreciable inductance at high carrier frequencies. The leads may be tuned to resonance at the carrier frequency to make them substantially'resistive, but immediately the 0b-. jectionable side band discrimination effect is intensified because the maximum phase shift takes place at frequencies adjacent to the frequency of resonance. An additional objection lies in the fact that for every change of carrier frequency, these leads must be retuned. The only circuits tuned to the carrier frequency in my invention are the plate and grid circuit resonant lines. Consequently, for each frequency at which side bandphase shifting takes place in the plate circuit resonant line, a voltage of that frequency having an opposite phase is impressed on the grid of the amplifier to effect complete neutralization of the voltage fed back by the grid-plate capacitance. In addition, an amplifier neutralized in the manner of my invention remains neutralized when the carrier frequency is changed because the neutralizing circuit is not resonant at carrier frequency.

Since the amplitude of theneutralizing voltage must be equal to the undesired feedback Voltage, the gain of the neutralizing tube must be reduced so that the net gain through the amplifier tube and back through the neutralizing tube is unity. This is accomplished by suitably selecting the operating potentials of the neutralizing tubes,

and by adjusting the value of the coupling capacitors 43, 5%, 59 and 6 i which may be variable, as shown. r

I have thus shown a system for neutralizing a radio frequency amplifier which provides efficient neutralization over a wide band of modulating frequencies, and which is substantially indethat a voltage is impressed on the respective grids of each of said first named thermionic devices which is equal in amplitude and opposite in phase to the voltage fed back tosaid grids by the grid-anode capacitance of said thermionic devices.

2. In a radio frequency-amplifier, a pair of thermionic amplifiers, an input and an output circuit connected to said thermionic amplifiers in push-pull relation, a pair of auxiliary thermionic devices for neutralizing said amplifiers and ineluding means respectively. coupling the grids of said auxiliary thermionic devices to the anodes of said first-named thermionic amplifiers, and means respectively coupling the anodes of said auxiliary thermionic devices to the grids of said first-named thermionic amplifiers, the amplification of said devices being so. adjusted that a voltage is impressed on the respective grids of each of said first-named thermionic amplifiers which is equal in amplitude and opposite in phase to the voltage fed back to said grids by the gridanode capacitance of said thermionic amplifiers.

3. A neutralized amplifier comprising input and output circuits, a thermionic amplifier having input and output electrodes coupled respectively to said input and output circuits for passing currents therebetween, an auxiliary thermionic tube having an input electrode coupled to said amplifier output electrode and an output electrode coupled to said amplifier input electrode, the sole function of said auxiliary tube being to provide a neutralizing voltage for said amplifier, and a load device coupled to said output circuit.

4. A neutralized radio frequency amplifier comprising input and output circuits, a thermionic amplifier tube having input and output electrodes coupled respectively to said input and output circuits, a load device coupled to said output circuit, a neutralizing tube connected in inverse parallel with said amplifier tube, said neutralizing tub-e being mounted adjacent said amplifying tube so that the grid of each tube is adjacent the plate of the other tube, whereby the length of the connections between said tube is minimized.

5. A neutralized radio frequency amplifier comprising input and output circuits, a pair of thermionic amplifier tubes having input and output electrodes connected between said input and output circuits, a neutralizing tube connected in inverse parallel with each of said amplifier tubes for applying to the input electrode of each amplifier tube a neutralizing voltage which is derived from and in phase opposition to the voltage of the output electrode of the same tube, and a load device connected to said output circuit.

6. In a radio frequency amplifier including input and output circuits and an amplifying circuit including a thermionic amplifier tube connected therebetween, means for neutralizing said amplifier tube to prevent self-oscillation, said means comprising a second similar thermionic tube connected in inverse parallel with said amplifier tube and not included in said amplifying circuit, the gain of said second tube being adjusted so as to impress a neutralizing voltage on said input circuit which is equal in amplitude and opposite in phase to the voltage impressed on said input circuit by the voltage in said output circuit.

7. A neutralized radio frequency amplifier comprising input and output circuits, a thermionic amplifier tube having input and output electrodes coupled respectively to said input and output circuit, a load device coupled to said output circuits, and a neutralizing tube connected in inverse parallel With said amplifier tube, the gain of said neutralizing tube being substantially less than the gain of said amplifier tube so that a neutralizing voltage is impressed on said input circuit which is equal in amplitude and opposite in phase to the voltage impressed on said input circuit by the voltage in said output circuit.

JOHN EVANS. 

